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I’m looking to make my own kiln coils and I wanted to know

  1. why are the coils coiled?
  2. When the coils are stretched too far apart they run cold and when they are close together they run hot. What is the reason for this if resistance doesn’t change whether a wire is coiled or straight?
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I wanted to know first off why the coils are coiled?

Suppose the wire is 10 m long. If you don't coil it, some of the heat it produces is "here" and some of the heat is 10 m away. Coiling it means you can heat a small area instead a long skinny area 10 m long.

when the coils are stretched too far apart thy run cold and when they are close together the run hot. What is the reason for this if resistance doesn’t change whether a wire is could or straight?

The temperature of the coils depends non only on how much heat they produce (\$I^2R\$) but also how much heat they lose to the environment. If you stretch the coil, it has an overall larger surface area over which heat is carried away by conduction and convection. If you compress the coil, it loses heat over a smaller area, and much of the heat produced by one turn of the coil actually heats the neighboring turns, rather than being lost to the environment.

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    \$\begingroup\$ well, technically the surface area is the same stretched or coiled...just the coiled wire is in warmer surroundings with which to lose heat to \$\endgroup\$ – DKNguyen Feb 12 at 19:04
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    \$\begingroup\$ @DKNguyen, not the surface area of the wire. The area of the surface through which it is effectively losing heat. \$\endgroup\$ – The Photon Feb 12 at 19:17
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    \$\begingroup\$ It’s neither the surface area of air or aperture of heat exit, but rather the surface velocity of air and humidity so some extent that affects heat removal rate with a drop in loss coefficient with rising velocity. Convection vs forced air vs chimney vs torroid effects \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 12 at 19:52
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    \$\begingroup\$ and if not that - storing say a few hundred meter of wire .... requires more room than a regular circuit board offers - so you have to "compact" it somehow .. and coiling it is a nice way to accomplish that \$\endgroup\$ – eagle275 Feb 13 at 10:50
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    \$\begingroup\$ A second reason for coiled is that the thermal expansion and contraction of the wire over the wide range of temperatures is significant. Managing the extension and sag in a straight wire can be more difficult than managing the extension and sag in a coiled, pre-tensioned spring. \$\endgroup\$ – Dave X Feb 13 at 15:35
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In addition to the accepted answer, coils also offer physical advantages in taking up the change in length when heated without sagging. The wire becomes brittle after use so the spring in the coil makes it easier to reroute into the channel in the firebrick if a coil pops out (heat the wire up when you do this).

I think sharp bends are subject to more strain with heating/cooling cycles so coils avoid those failure spots.

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In addition to all of the correct answers: coiling a wire does change its inductance, which is something like resistance except it only affects the flow of AC current, not DC. This isn't the reason for your coils (which are fed from DC or 50/60 Hz AC — at those frequencies, the inductance isn't enough to matter much, and doesn't contribute to heating). However, it is a reason why you will see coils in other kinds of electronics, including radios, motors, and power supplies. They're not trying to keep heat in (usually they want to get rid of as much heat as possible), but they are trying to regulate the flow of current by storing energy in magnetic fields.

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  • \$\begingroup\$ I had read a bit about inductance and couldn’t really understand it. However your comment helped tremendously on it. Thank you \$\endgroup\$ – Wes Feb 13 at 18:03
  • \$\begingroup\$ And in motors the windings aren't just for their inductance per se, but instead to generate a magnetic field which exerts mechanical force. \$\endgroup\$ – Peter Cordes Feb 14 at 13:30
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One more addition: most wires DO change their resistance when subject to mechanical stress and deformation. The resistance generally goes UP for almost any deformation and the effect is negligible in most cases.

It is definitely negligible for a heating coil, but can be an important source of error in a current measurement shunt.

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    \$\begingroup\$ For copper that effect is of the order of 1-2% increase. Some of it will come back with annealing. \$\endgroup\$ – Spehro Pefhany Feb 14 at 1:47
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    \$\begingroup\$ If you didn't concentrate the heat in a smaller area by coiling (and surrounding it with a kiln) you also wouldn't have such a big effect from the temperature coefficient of resistance. Getting hotter raises the resistance some. (Very big effect in an incandescent light bulb, I assume less so in heating coils.) \$\endgroup\$ – Peter Cordes Feb 14 at 13:33
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    \$\begingroup\$ @PeterCordes heating coils are made of alloys (nichrome, kanthal) pretty much immune to this effect. It is within 10% from room temperature to almost melting temperature (vs 10-fold increase of the resistance for incandescent bulbs). That's why you don't need to deal with starting current as you do for incandescent lights or motors. \$\endgroup\$ – fraxinus Feb 14 at 15:23
  • \$\begingroup\$ Thanks, didn't realize it would be that small an effect but that makes sense. I guess a big TC over a physically larger element could lead to runaway hotspots even more easily than in a filament. \$\endgroup\$ – Peter Cordes Feb 14 at 16:01
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    \$\begingroup\$ @PeterCordes the filament in the light bulb is cooling itself mainly by radiation and the T^4 part in the Stefan-Boltzman law makes it rather hard to create a hotspot. OTOH, that's how insulated power cables sometimes die when they are "mildly" overloaded for a while. \$\endgroup\$ – fraxinus Feb 17 at 13:12
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As the other answers point out, resistance doesn't change with shape. In a kiln though, like a light bulb, much of the heat is lost radiatively. in a coil, the infra-red radiation emitted towards the center of the coil will simply be reabsorbed the wire the other side, hence as the wire is coiled, the effective surface area for IR emission goes down, hence temperature goes up. Resistance does increase as temperature goes up, for most if not all metals. This gives something of a regulatory effect limiting the temperature.

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